Revealing mistake: When the first baby is born, Monica and Chandler are asked by the doctor if they would like to cut the umbilical cord. However, just before this line is delivered, the baby is moved and its navel is visible.
The One With Ross' New Girlfriend - S2-E1
Other mistake: After Rachel takes the phone from Ross and hangs up on Julie, when she's sitting at the table there's a bottle of Evian water in front of Monica with part of its brand name covered - the letters "e" and "n" are whited out leaving "via" on the bottle.
The One After Joey And Rachel Kiss - S10-E1
Visible crew/equipment: After the gang leaves Rachel's room, right after she asks Joey "Are they right?", if you look above Joey's head, for a split of a second you will see a boom mic. (00:09:25)
The One Where Ross Got High - S6-E9
Factual error: A really picky one: When Ross looks at the recipe book, he says Rachel's made 'half a Shepherd's Pie'. Technically, a Shepherd's Pie is made with lamb, whilst it's a Cottage Pie that's made with beef (although the two names are often interchanged).
The One With All The Thanksgivings - S5-E8
Continuity mistake: When Phoebe is trying to turn on the TV, her necklace moves without her touching it. (00:00:15)
The One With The Metaphorical Tunnel - S3-E4
Continuity mistake: The position of the lid of Janice's box of contact paper changes repeatedly and after she tells Chandler she doesn't know what to say, she begins to slide the roll of paper back into the box. In the shot from behind, the box is closed. (00:09:50)
The One With The Apothecary Table - S6-E11
Continuity mistake: When Chandler says "Oh, just like the apothecary table of yore," Monica turns her head toward him twice. (00:02:30)
The One With The Fake Party - S4-E16
Continuity mistake: When Rachel is in her bedroom dressed as a cheerleader with Joshua, the sleeve of her sweater on her right arm keeps moving up and down. (00:15:00)
The One After Joey And Rachel Kiss - S10-E1
Continuity mistake: At the end of this episode, Ross enters the door of Joey's apartment and finds Rachel and Joey kissing. Rachel says "I'm so sorry". The next episode, The One When Ross Is Fine, shows the same scene but it has been changed: Rachel never says "I'm so sorry". Joey starts talking when Ross enters the room.
The One With The Birth Mother - S10-E9
Audio problem: When Monica and Chandler are talking about telling Erica, when Monica says "Technicality!" It doesn't match with her lips. (00:15:30)
The One In Massapequa - S8-E18
Continuity mistake: In The One In Massapequa, Jack Gellar is eating food off a stick. When he finishes he holds the stick while Rachel asks Jack if he has any pearls of wisdom. We can see the stick over Judy's shoulder. In the next shot the stick is gone from his hand, then we see Judy put it down on a table, then it reappears in his hand, before disappearing for good. (00:05:05)
The One With Rachel's Crush - S4-E13
Continuity mistake: At the start of this episode when Rachel comes in, you can see a bit of wooden floor by the door (carpet's visible in other shots). However, Monica doesn't pull up the carpet until later in the episode, after the apartment's tidier. (00:02:10)
The One With Ross's Grant - S10-E6
Visible crew/equipment: When Dr.Hobart enters the meeting room to conduct the grant interviews, markers can be seen on the floor around the first chair's wheels as well as in front of the table. (00:12:25)
The One With The Worst Best Man Ever - S4-E22
Other mistake: Given her vegetarianism, and her disdain for fur in another episode, there's no way Phoebe would have wanted a pair of leather pants.
Suggested correction: It's your opinion that Phoebe wouldn't have wanted leather pants, but that's not true of all vegetarians. Natalie Merchant was known for saying she was a vegetarian but still wore leather shoes. I was a vegetarian who would never have worn fur and still wore leather shoes.
The One With Rachel's Going Away Party - S10-E16
Plot hole: In this episode Chandler finds furry black handcuffs, and everybody says that they have never seen them before. How is that possible considering all the people that have moved in and out of that room? How come nobody noticed them when the room and closet were painted green after being pink?
The One With Rachel's Other Sister - S9-E8
Character mistake: While watching the parade near the beginning, Chandler gets excited because there's a band from Muskogee, OK, which is supposedly only four hours from Tulsa, where he works. It's actually less than an hour away from Tulsa.
The One Where Monica Gets a Roommate - S1-E1
Revealing mistake: As Monica is walking to her room and steps on Paul's watch, if you look carefully, you can see that she just steps on the band. That would not damage the watch. (00:26:45)
Factual error: The makeup on Rachel's face "can't be covered up with makeup," and "won't come off," we learn in The One In Vegas, Part 2. So it's not a surprise that she still wakes up with it on (and so does Ross.) but an hour or so later, when she's at breakfast, it's not there. An hour or two won't make the magical difference that 8+ hours didn't.
The One Where Joey Moves Out - S2-E16
Other mistake: When Rachel is showing Phoebe her heart tattoo, the tattoo isn't dressed and covered, nor is the skin around it red or swollen, despite the fact she just finished getting it. (00:14:09)
The One Where Joey Dates Rachel - S8-E12
Factual error: When Chandler is explaining that he played Ms. Pac Man all day, the Ms. Pac Man machine is unplugged, and then plugged back in. Immediately upon being plugged back in, we hear the Ms. Pac Man theme song being played. No version of a Ms. Pac Man stand-up arcade ever played the theme song upon powering on. Instead, it would silently go into "demo mode." The only way the stand-up arcade version of Ms. Pac Man would ever play its theme song was after two events occurred in order: 1. A coin was dropped in the slot (or a button was pressed on the inside of the unlocked coin drawer), and then 2. Someone pressed the '1 player' or '2 players' button to start the game. It is clear that neither of these things has happened in the scene by the time the music starts playing. (00:11:50)
